1. Field of the Invention
The invention relates to a method of operating a kiln system consisting of a firing kiln heated with pulverized fuel, an air cooled cooling device for the fired material and a crusher. The invention further relates to such a kiln system. It is especially intended for the coal dust firing of rotary kilns, such as those employed for the manufacture of cement clinker, fired lime, sintered dolomite and sintered ore. In this process, at least part of the cooling air is led to the firing kiln as secondary air after it flows through the cooling device. In addition, the pulverized fuel from the crusher is pneumatically conveyed to a separator, from where, on the one hand, the greatest portion of the pulverized fuel is drawn to the burner, and on the other hand, the conveying air is drawn off together with a small, unseparated portion of the pulverized fuel.
2. Prior Art
It is known to operate this type of system with a so-called blow in crusher. This means that the pulverized fuel, together with the conveying air which carries it out of the crusher, is led directly to the burner of the firing kiln. The conveying air is thus the primary air. Because the quantity of the conveying air must be determined with regard to its conveying function, this air quantity is much greater than the desired quantity of primary air. The latter should namely be held as small as possible with regard to flame formation, so that for reasons of heat economy the greatest possible proportion of the total air can be supplied from the highly heated secondary air. In the conventionally designed systems the primary air proportion in this respect is more than 18% of the total air, while a primary air proportion of approximately 8% would be desirable. Furthermore, the conventional method has the disadvantage that the conveying air, depending on the water content of the fuel, can contain a large amount of moisture, which lowers the absolute oxygen content. With regard to flame formation, however, a high oxygen content in the primary air is desirable.
In the so-called indirect blow-in method, which has become known through open prior use, the pulverized fuel is led with the conveying air into a separator. A portion of the dust-laden conveying air drawn from the separator is circulated back into the crusher, while another portion is used as the primary air and to convey the pulverized fuel to the burner. The primary air quantity is indeed decreased by this method and the heat economy is improved. Simultaneously, however, the relative moisture content of the primary air increases, because the entire moisture from the fuel is taken exclusively with the primary air out of the air circulatory system of the crusher. Furthermore, the absolute primary air quantity cannot be sufficiently lowered with regard to the drawn off quantity of moisture as would be desirable for heat economy.
In a third conventional method with a central crushing apparatus the conveying air is released to the atmosphere after the fuel dust has been separated out in filters. The selection of type and quantity of the primary air thus becomes independent of the conveying air. However, on the one hand this had disadvantages in heat economy, since the conveying air is generally heated to assist its drying function, and on the other hand filter systems cause increased danger of smoldering fire and explosion.